We studied the mass-related aerosol properties, simultaneously at two sites at the urban roof top level in the same city. No systematic influence of the wind vector on the difference in the aerosol concentrations between the two locations could be found. These results are compared with results from a second, similar experiment over a larger distance including one urban and one rural site. Surprisingly, we could not detect a tendency which would indicate that sampling air at distance in the order of 1 km would be less affected by the heterogeneity than sampling distanced in the order of 10 km apart. On the contrary, the results suggest that mass-related properties at two sites in the same city are not necessarily more similar than at an urban and a rural site outside the city. These results stress the limited horizontal homogeneity of urban atmospheric aerosol. As a conclusion it is suggested that single-site measurements of mass-related aerosol properties should be considered to be representative for an area smaller than 1 km(2) on size.